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Fricatives are consonants with the characteristic that air escapes through a narrow passage and makes a hissing sound. The dental fricatives are sometimes described as if the tongue were placed between the front teeth, and it is common for teachers to make their students do this when they are trying to teach them the sound. The thing is, however, that the tongue is normally placed behind the teeth; the air escapes through the gaps between the tongue and the teeth. There is a distiction between fortis (unvoiced) fricatives, as in the word “thin”, and lenis (voiced) fricatives, as in “thus”. (Roach 2003)

(Mark Roach,. English Phonetics and Phonology. Cambridge: CUP, 2003. Adaptado)